1. Field of the Invention
The present invention relates, in general, to high pressure sodium lamps. In particular, the invention relates to a saturated vapor pressure type sodium lamp having an arc tube wherein an excess amount of mercury and sodium are provided in an amalgam state, and almost all amalgam is accumulated at the coolest portion of a metal tube attached to one of the ends thereof. A proper amount of starting rare gas also is sealed in the arc tube.
2. Description of the Prior Art
High pressure sodium lamps have, in general, a high efficiency. The sodium lamps are widely used as light sources for lighting wide areas, e.g., parking places, open spaces, etc.
Recently, there has been a greater demand for high efficiency and large luminous flux arc lamps. Such a high efficiency high pressure sodium lamp may be obtained by increasing the diameter of the arc tube of the lamp (hereinafter referred to as a conventional design technique). In general, the efficiency of the high pressure sodium lamp depends on the inner diameter of the arc tube, the amalgam sodium ratio in the tube and the electric field strength of the tube. The amalgam sodium ratio is the ratio of the sodium in the amalgam to the amalgam which are sealed in the arc tube of the high pressure sodium lamp. The electric field strength is obtained by dividing the voltage applied between a pair of electrodes of the lamp by the length of the arc tube.
In conventional high pressure sodium lamps, metal vapor having a proper amount of sodium and a prescribed amalgam sodium ratio which correspond to the inner diameter of an arc tube, as shown in FIG. 1, is sealed in each arc tube. Furthermore, each lamp having the above-described arc tube is operated under the prescribed electric field strength corresponding to the inner diameter of the arc tube, as shown in FIG. 2. However, if the diameter of the arc tube is increased to achieve a large luminous flux, the lamp efficiency initially increases to a maximum value when the inner diameter of the arc tube is about 12 mm, and then sharply decreases as the inner diameter is increased further, as indicated by the dotted curved line in FIG. 3. This is because, with the increase of the inner diameter of the arc tube beyoned 12 mm, the quantity of sodium vapor, located near the wall of the arc tube, which absorbs D-resonance rays from the arc increases, while the heat loss from the end portions of the arc tube decreases.
As described above, if the conventional design technique is applied to an arc tube having a large inner diameter, the lamp efficiency sharply decreases when the inner diameter is greater than 12 mm. Therefore, the efficiency of the high pressure sodium lamp is adversely affected, and it is difficult to put a high efficiency sodium lamp to practical use.